There are many instances where cells or microorganisms are measured in liquids or solids. Samples such as air, water, food, beverages and clinica samples are: routinely tested with various microbiological methods. It is essential that these tests are performed accurately, reliably and that results are comparable between samples and between laboratories.
The testing methods used vary greatly but all involve some manipulation of the sample. Examples include flotation, pipetting, centrifuging filtering, or homogenising. Losses of the sample and losses of cells or microorganisms will occur during these manipulations. These losses will vary between different types of samples. Some methods may also involve various pre-treatments such as heating, freezing, shaking, mixing or pH adjustment. These treatments may also cause losses of the cells or microorganisms. All testing methods involve a detection process and may also involve enumeration of the particles recovered. This may be a simple process such as counting colonies on an agar plate or it may be a complex analytical process such as flow cytometry or immunomagnetic separation. All these detection and enumeration processes will have inaccuracies that will effect the result of the test.
Quality control (QC) is routinely performed to check the accuracy of a test method and to determine what losses of cells or microorganisms are occurring during the testing process. The QC process often involves the analysis of a sample that has been seeded with specific cells or microorganisms. The number of cells or microorganisms that were seeded into the sample are then compared to the number detected.
The QC process relies on the availability of control samples that contain known numbers of cells or microorganisms. These control samples must be reliable and the numbers within each sample must be accurate. Such control samples are difficult to prepare, are often not stable and are not widely available.
Water is routinely tested for the presence of Cryptosporidium and Giardia. The testing method involves numerous processes in which organisms can be lost (Vesey et al., 1994a). It is, therefore, important to perform stringent quality control the performance of the methodology. Quality control can involve analysing a standard that contains a known number of cysts and oocysts. Preparation of standards using conventional techniques such as pipetting result in inaccurate preparations (Reynolds et al, 1999). Furthermore, preparations are not stable. Deterioration and clumping of the microorganisms occurs over time.
Flow cytometry has been shown to be useful for the preparation of accurate control material (Reynolds et a., 1999), however the method is tedious and time consuming and therefore, not suitable for large-scale production.
Cooling water and water distribution systems are routinely tested for the presence of Legionella. The testing method involves concentrating the bacteria in the water sample by centrifugation or filtration and then detecting the Legionella bacteria by either a culturing process, immunological techniques or molecular methods. Use of the culturing process is limited because it takes between 5 and 10 days to obtain a result.
It is essential that the immunological and molecular methods are stringently quality controlled because both methods are susceptible to false negative results. Quality control can involve analysing a standard that contains a known number of Legionella cells. However, accurate standards are difficult to prepare and are not stable. Preparations can be prepared using flow cytometry to analyse the light scatter of the cells and to sort the cells. If live Legionella cells are to be sorted then a aliquoting method that does not create aerosols, such as piezo cappillary dispensing, would be required to overcome biohazard risks.
A need accordingly exists for an automated, accurate, convenient and cost-effective process for producing control samples containing known numbers of cells or microorganisms.
The invention allows the automated preparation of control samples containing predetermined numbers of small particles such as microorganisms or cells. These samples containing the known number of microorganisms or cells may be gamma-irradiated or otherwise treated to prolong the stability and shelf life of the samples. A percentage of the samples may, be tested to confirm the number of microorganisms or cells in each sample.